Germanium (Ge) is a semiconductor that has better electrical properties than silicon (Si). However, germanium oxide (for example, GeO2) is unstable. Therefore, germanium is hardly used as a semiconductor material forming a MOSFET (Metal Oxide Semiconductor Field Effect Transistor).
Non-patent documents 1 and 2 disclose that an interface condition between a germanium substrate and a germanium oxide film is improved when high pressure oxygen gas is used during forming the germanium oxide film on the germanium substrate.